Method and apparatus for forging gear teeth

ABSTRACT

A forged gear and a method of manufacture thereof. The forged gear comprises a bevel gear with a plurality of teeth, each tooth having a tip disposed between two faces and two ends, the two faces extending in a substantially longitudinal direction of the tooth, wherein the tip of each tooth has at least one substantially crowned portion between the two ends along the longitudinal direction. The method of manufacture comprises forging the gear from a blank such that during the forging operation a substantial portion of the length of each forged tooth tip does not contact the die cavity.

TECHNICAL FIELD

The present invention relates to a gear tooth form adapted for forging and to a method and apparatus for forging toothed gears, and in particular for forging bevel ring gears. Whilst the embodiment of the present invention is described with reference to bevel ring gears for motor vehicle drive axles, the present invention is also suitable for other types of gears.

BACKGROUND

Bevel ring gears and pinion gears are well known and commonly used in power transmission applications. Bevel ring gears and pinion gears have various known tooth forms including straight, spiral and hypoid types.

The majority of bevel ring gear teeth are manufactured by gear cutting. Such gears are machined from a forged and/or ring rolled blank that is turned prior to gear cutting. The blank has the form of the finished gear minus the tooth gaps on one side and has a number of threaded holes located in the opposite side or mounting face of the gear.

The tips of the teeth in conventional machined bevel gears lie on the surface of a cone. The inner and outer end surfaces of the teeth likewise lie on conical surfaces. The axes of the tip cone and the inner and outer tooth end surface cones are co-axial.

The gear cutting process is typically a generating or hobbing process, as described by Shtipelman-“Design and Manufacture of Hypoid Gears” ISBN 0-471-03648-X, and produces a sharp edged tooth form that may need to be be-burred.

The flanks (or working surfaces) of the gear teeth, are bounded by the tip cone, and the inner and outer tooth end surface cones. The form of the flanks is determined by the gear cutting process, for example straight cut, spiral or hypoid, and is of a form required for conjugate action with the mating pinion.

The conjugate tooth form extends over the entire surface of the tooth flank. However, it is known in the art that it is desirable that the contact between mating gear teeth occurs only over a defined area on each of the two tooth flanks as described in Shtipelman-“Design and Manufacture of Hypoid Gears” ISBN 0-471-03648-X. This area of contact is referred to herein as the “contact zone”.

In lightly loaded gears such as the bevel gears used as side gears in automobile rear axle differentials, the gear teeth are often finished to net shape by forging or by forging and cold coining as described in Praezisionsgeschmiedete Getriebeteile. Umformtechnik, vol. 33 (1999), issue 4, pp. 16-18. The location and extent of the contact zone in this case is determined by the design of the forging die and the elastic deflections of the forging die due to the pressures to which it is exposed during forging.

In highly loaded gears such as hypoid bevel gears, the gear teeth are mainly manufactured by gear cutting, hence the location and extent of the contact zone is determined by the gear cutting machining process and by the gear tooth finishing operation which may be either grinding or lapping as described in U.S. Pat. No. 5,088,243 (Krenzer), U.S. Pat. No. 5,255,475 (Kotthaus), and U.S. Pat. No. 4,788,476 (Ginier). The finishing operation is carried out after the gear has been hardened. The finishing process is also adapted to produce relief on the tooth flanks where contact with the mating gear is not desired through removal of excess material.

The physical extent of the contact zones under load is also a function of the tooth load and the mechanical stiffness of the gear teeth and the body of the gear on which the teeth are located; the stiffness of the mating pinion gear teeth and the body on which they are mounted; and the stiffness of the assembly in which the ring gear and pinion are mounted. The gear designer determines the location and extent of the contact zones to achieve appropriate tooth meshing and acceptable outcomes for tooth bending and contact stresses.

Various attempts have been made to forge near net or net shape ring gears with straight, spiral or hypoid teeth as described in U.S. Pat. No. 4,856,167 (Sabroff et al.) and U.S. Pat. No. 4,050,283 (Schober) and European Patent No. 1068912 (Kotani K.K). There are no known examples of forged net shape or near net shape ring gears in large volume production. In all known cases the teeth so formed resemble machine cut teeth, with small corner radii where the tooth ends and the tooth tips blend with the tooth flanks.

One reason such forged net shape or near net shape ring gears have not been successful in volume production has been the inability to achieve commercially acceptable die life. This is particularly so in closed die forging, where it is critical to control the forging process to prevent over-pressurising the forging die tooth cavities. This is less of a problem with orbital or open die forging, but such processes are not suitable for high precision net shape forging and they produce a substantial amount of flash that has to be machined off after forging.

The failure mechanisms in forging dies vary according to whether the gear is forged cold, warm or hot. In cold and warm forging, the failure mechanisms are abrasive wear and low cycle fatigue as described in J. Groenbaek et al, “Optimisation of Life Performance and Net-Shape Capability of Cold-Forging Tools through Minimisation of Micro-Plastic Phenomena”-Advanced Technology of Plasticity, Vol. 1, Proceedings of the 6th ICTP, Sep. 19-24, 1999, pp. 243-252. In hot forging the failure mechanisms are abrasive wear and thermal fatigue as described in “Reibung und Verschleiss beim Schmieden, ISBN 3-8265-4373-4, pp. 69-94).

In either cold, warm or hot forging, die mechanical stresses may be minimised, inter alia, by appropriate design of the tooth cavity with particular attention to the design of the tooth cavity corner and root radii including where the tooth cavity blends in with the top surface of the die. The tooth die root radii correspond to the forged gear tooth tip edge radii. If the tooth tip edge radii are too small, and the same radii are used as the die tooth root radii, the degree of stress concentration is very high and failure of the tooth die in low cycle fatigue may be expected. The same can be said about the end corners of the tooth die cavity, which correspond to the tooth end corner radii.

To facilitate the flow of metal into the forging die tooth cavities and to limit wear and tear on the tooth die, it is necessary to apply generous radii to the top edges of the tooth cavities. This results in a blended radius at the base of each forged tooth where the tooth form merges with the ring form of the gear. These radii also serve to reduce stress concentration in the roots of the gear teeth under load. Such corner radii are therefore highly desirable from the points of view of both the gear designer and the forging die designer, and should be as large as possible without causing interference between the mating gear teeth.

However, if the forged tooth is basically the same form as that of a machine cut gear with small corner radii at the ends and in the roots of the tooth cavities, the degree of stress reduction achievable in the tooth die is severely limited, as is the ability to achieve significant improvements in tooth die life.

With axially symmetric dies, beneficial reductions in die peak stresses may be obtained by using shrink rings or pre-stressed containers (J. Groenbaek et al, “Optimisation of Life Performance and Net-Shape Capability of Cold-Forging Tools through Minimisation of Micro-Plastic Phenomena”-Advanced Technology of Plasticity, Vol. 1, Proceedings of the 6th ICTP, Sep. 19-24, 1999, pp. 243-252). However, with dies having complex tooth cavity geometry such as in hypoid or spiral bevel gears, the reduction in peak stress levels achievable by these means is not sufficient to ensure acceptable tooth die life if the forged tooth form is basically the same as that of a machine cut gear having small tooth end or tooth tip edge radii.

The problem is further exacerbated by attempting to fully fill the die cavity in net shape or near net shape forging when using a closed die. It is known in the art that once a closed die cavity is fully filled any further increase in forging load will simply increase the hydrostatic pressure in the material being forged, and will further dramatically increase the mechanical stresses in the closed forging die.

It is also known in the art that the accuracy of closed die forging increases as the forging load (and hence die pressures) is reduced (E. Körner et al, “Computer-Aided Development of Precision Formed Parts”, International Conference on New Developments in Forging Technology, Fellbach, Germany, Jun. 3-4 2003, page 277, K. Siegert ed). In order to achieve better die life and at the same time improve accuracy in closed die forging, it is desirable to reduce both forging pressures and die stress concentrations.

Forging loads and corresponding die pressures and stresses may be reduced by deliberately introducing a low pressure zone around the die cavity. This is commonly done in open die forging by provision of one or more flash gutters along the parting line of the forging dies. However this produces a large amount of flash that has to be removed in a subsequent machining operation. This is wasteful of material, introduces an extra manufacturing operation and is therefore undesirable.

Provision of flash gutters is not an option in closed die forging, particularly in net or near net shape forging of precision gear teeth. An alternative means of limiting forging die pressures is therefore required for net or near net shape forging of gear teeth.

In prior art attempts to forge gear teeth, the gear and forging designers have attempted to design gear teeth that resemble machine cut gear teeth with small corner and tip radii. This has resulted in the disadvantages associated with the earlier mentioned prior art, such as stress concentrations, tooth die fatigue etc.

The present invention seeks to ameliorate at least some of the disadvantages of the prior art with a view to achieving substantially increasing die life, with particular reference to the forging of gear teeth to net shape or near net shape in a closed die.

SUMMARY OF INVENTION

In a first aspect, the present invention consists of a forged bevel gear having a plurality of teeth, each tooth having a tip disposed between two faces and two ends, said two faces extending in a substantially longitudinal direction of said tooth, characterised in that said tip has at least one substantially crowned portion between said two ends along said longitudinal direction.

Preferably, said crowned portion is forged as a smooth continuous surface. Preferably, said substantially longitudinal direction of said tooth lies along a substantially radial direction. Preferably, said radial direction is curvilinear. Alternatively, said radial direction is straight.

Preferably, at least 25% of the length of said tip is crowned. More preferably, at least 50% of the length of said tip is crowned.

In one preferred embodiment, said at least one crowned portion is two crowned portions. Preferably, a central portion is located between said two crowned portions, and said central portion is any one of a substantially flat portion, a slightly rounded portion or a rounded portion. Alternatively, a substantially concave portion is centrally located between said two crowned portions.

In another preferred embodiment, at least one flat is machined onto said gear between said at least one substantially crowned portion and one of said ends.

In a second aspect, the present invention consists of a method of forging a gear from a blank, said gear having a plurality of teeth, each said tooth having a tip disposed between two faces and two ends, said two faces extending in a substantially longitudinal direction of said tooth, said method comprising the steps of:

-   -   i) placing said blank within a die means comprising a cavity         having a shape that is the near obverse of said teeth; and     -   ii) forging said blank within said cavity to form said teeth         thereon;

characterised in that during said forging, a substantial portion of the length of each said tip does not contact said cavity.

Preferably, wherein said substantial portion is at least fifty percent of the length of said tip. More preferably, said substantial portion is at least sixty percent of the length of said tip. Still more preferably, said substantial portion is at least seventy-five percent of the length of said tip.

Preferably, said tip has at least one substantially crowned portion between said two ends along said longitudinal direction.

Preferably, said gear is a bevel gear.

In a third aspect, the present invention consists of a forged gear manufactured by the methods of the second aspect of the present invention.

In a fourth aspect, the present invention consists of a forged bevel gear having a hypoid configuration for use in a vehicle power transmission, said gear comprising a plurality of teeth, each tooth having a tip disposed between two faces and two ends, said two faces extending in a substantially radial direction, characterised in that said tip has at least one substantially crowned portion between said two ends along said radial direction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a prior art machine cut hypoid bevel gear.

FIG. 2 depicts a plan view of the bevel gear shown in FIG. 1.

FIG. 3 depicts a cross-sectional view of the bevel gear shown in FIG. 2 through III-III.

FIG. 4 a is a perspective view along arrow A of a single tooth of the bevel gear shown in FIG. 1.

FIG. 4 b is an end view along arrow B of the single tooth shown in FIG. 4 a.

FIG. 4 c is an elevational view along arrow C of the single tooth shown in FIG. 4 a.

FIG. 5 a is a perspective view of a first embodiment of a single forged hypoid tooth where the entire tip of the tooth has not contacted the die during forging in accordance with the present invention.

FIG. 5 b is an end view along arrow B of the single forged hypoid tooth shown in FIG. 5 a.

FIG. 5 c is an elevational view along arrow C of the single forged hypoid tooth shown in FIG. 5 a.

FIG. 5 d is an enlarged view of the tooth portion shown in circle D of FIG. 5 b.

FIG. 5 e is an enlarged view of the tooth portion shown in circle E of FIG. 5 c.

FIG. 5 f is an enlarged view of the toothed portion shown in circle F of FIG. 5 c.

FIG. 5 g is a schematic profile of the tip of a second embodiment of a single forged hypoid tooth according the present invention showing convex crowning of the tooth tip.

FIG. 6 a is a perspective view of a third embodiment of a single forged hypoid tooth where the centre portion only of the tooth tip has contacted the die during forging in accordance with the present invention.

FIG. 6 b is an end view along arrow B of the single forged hypoid tooth shown in FIG. 6 a.

FIG. 6 c is an elevation view along arrow C of the single forged hypoid tooth shown in FIG. 6 a.

FIG. 6 d is an enlarged view of the tooth portion shown in circle D of FIG. 6 b.

FIG. 6 e is an enlarged view of the tooth portion shown in circle E of FIG. 6 c.

FIG. 6 f is an enlarged view of the tooth portion shown in circle F of FIG. 6 c.

FIG. 6 g shows an enlarged end view of a tooth portion of fourth embodiment of single forged hypoid tooth in accordance with the present invention, viewed in the same manner as FIG. 6 d.

FIG. 6 h is an enlarged elevation view of the tooth portion shown in FIG. 6 g, viewed in the same manner as FIG. 6 e.

FIG. 6 j is an enlarged elevation view of the tooth portion shown in FIG. 6 g, viewed in the same manner as FIG. 6 f.

FIG. 6 k is a schematic profile of the tip of the single forged hypoid tooth shown in FIG. 6 a showing two convex crowned tooth tip portions juxtaposed with a flat tooth tip portion.

FIG. 7 a is a perspective view of a fifth embodiment of a single forged hypoid tooth in accordance with the present invention where two portions only of the tooth tip have contacted the die during forging.

FIG. 7 b is an end view along arrow B of the single forged hypoid tooth shown in FIG. 7 a.

FIG. 7 c is an elevation view along arrow C of the single forged hypoid tooth shown in FIG. 7 a.

FIG. 7 d is a schematic profile of the tip of a sixth embodiment of a single forged hypoid tooth in accordance with the present invention showing two convex crowned tooth tip portions juxtaposed with a concave tooth tip portion

FIG. 7 e is a schematic profile of the tip of a seventh embodiment of single forged hypoid tooth according to the present invention showing two convex crowned tooth tip portions juxtaposed with two flat tooth tip portions that are in turn juxtaposed with a concave tooth tip portion.

FIG. 8 a is a perspective view of an seventh embodiment of a single hypoid bevel gear tooth in accordance with the present invention having a smooth curved form adapted to minimise die stresses during forging.

FIG. 8 b is an end view along arrow B of the single hypoid bevel gear tooth shown in FIG. 8 a.

FIG. 8 c is an elevational view along arrow C of the single hypoid bevel gear tooth shown in FIG. 8 a.

FIG. 8 d is an enlarged view of the tooth portion shown in circle D of FIG. 8 b.

FIG. 8 e is an enlarged view of the tooth portion shown in circle E of FIG. 8 c.

FIG. 8 f is an enlarged view of the tooth portion shown in circle F of FIG. 8 c.

FIG. 8 g shows sections (i), (ii), (iii), (iv), and (v) viewed along arrow B from the end of the single hypoid bevel gear tooth shown in FIG. 8 a.

FIG. 8 h shows enlarged views circles G, H, J, K and L of FIG. 8 g.

FIG. 9 a is a perspective view of the prior art machine cut hypoid ring gear of FIG. 1.

FIG. 9 b is a perspective view of a forged hypoid ring gear in accordance with the present invention having a smooth curved tooth form similar to that of the eighth embodiment shown in FIG. 8 a adapted to minimise die stresses during forging.

BEST MODE OF CARRYING OUT THE INVENTION

The present invention will be described primarily in relation to hypoid bevel ring gears for motor vehicle right angle drive axles, it being well understood that the present invention is equally well suited to bevel gears having other tooth forms such as straight or spiral teeth, and the present invention is well suited to bevel ring gears in other applications. The method of the present invention is also suitable for producing other types of bevel gears, such as pinions. The tooth form modifications described herein with reference to bevel gear teeth may be applied to other types of gear teeth, such as straight and helical spur gears.

FIGS. 1, 2 and 3 depict a typical prior art machine cut hypoid ring gear 1 such as is used in a motor vehicle right angle drive axle. Gear 1 comprises a mounting bore 2, a mounting face 3 on the back of gear 1, threaded holes 4, gear teeth 5, conical outer surface 6, inner step 7, and conical inner surface 8. Gear teeth 5 comprise flanks 9 and 10, tips 11, inner ends 12, outer ends 13 and roots 14. Mounting bore 2 and mounting face 3 are the mounting surfaces of the gear. The method and apparatus of the present invention are also suited to bevel ring gears with variations to the shape shown in FIGS. 1, 2 and 3, such as gears having a cylindrical outer surface 6 instead of conical surface 6, or gears not having inner step 7. Although gear 1 is shown with chamfers 50 on the inner ends of teeth 5, it is understood that the following pertains also to gears without such chamfers and to gears where both ends of the teeth may be chamfered. Bevel ring gears similar to the type depicted in FIGS. 1, 2 and 3 may alternatively be formed in a prior art forging die of the type shown in FIG. 23 of International Patent Publication No. WO 02/078876 entitled “Forging Method and Apparatus”.

FIG. 4 a depicts a single machine cut hypoid gear tooth of ring gear 1 shown in FIG. 1. The edges of the tooth are defined by inner edges 15 and 28 (see FIG. 1); outer edges 16 and 17; top edges 18 and 19, 23 and 30; tooth chamfer edges 23, 24, 27 (see FIG. 1) and 29 (see FIG. 1); end corner radii 20, 21, 22 and 31 (see FIG. 1); and lines 26 that denote the bottom of tooth roots 14. Note that in some gears, the chamfered tooth corner defined by edges 23, 24, 27 and 29 are not required.

FIGS. 4 a, 4 b and 4 c, being a machine cut gear tooth with sharp corners, represents the maximum volume tooth form for this particular gear. If this gear tooth were to be manufactured by forging, it would be necessary to apply a radius to all sharp edges in order to make die machining practical and to reduce the effects of stress concentration in the corresponding corners of the tooth die cavity. Apart from the before described modification of sharp edges the not shown tooth die cavity would be the exact obverse of the machine cut gear tooth 5.

FIG. 5 a depicts a first embodiment of a forged gear tooth 205 and is based on the machine cut gear tooth 5 shown in FIG. 4 a. The outline of the “parent” machine cut gear tooth 5 is represented by the chain dotted lines 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 230, 232 and the solid lines 226, the latter representing the loci of the lowest points in root radii 214.

For the purposes of this document, the length 270 of gear tooth 205 is measured in the curvilinear direction along the bottom of root radius 214 (ie: along line 226, see FIG. 5 c).

Curvilinear direction t lies in reference surface S of FIG. 1. Surface S is a curved vertical surface and intersects gear 1 in line P-P which is coincident with lines 26 in the roots of gear teeth 5 (see also FIG. 2). Direction z is at right angles to curvilinear direction t and is parallel to the root 14 of gear tooth 5 (see FIG. 1). As shown in FIG. 3, direction z is in general not parallel to the axis Z of gear 1.

The corresponding tooth die cavity 237 is also represented by the chain dotted lines 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 230, 232 and the solid lines 226, where it is understood that those lines representing sharp edges in the machine cut gear 5 tooth are replaced by small, generally less than 1 mm, radii that blend smoothly with the other surfaces of the tooth die cavity and the top surfaces of the tooth die.

The extent of forged gear tooth 205 that is common with flank 9 of the parent machine cut gear tooth 5 is bounded by lines 233 and 226. The extent of forged gear tooth outer end 213 that is common with the tooth outer end 13 of parent machine cut gear tooth 5 is bounded by lines 235 and 236. Tooth flank 209 is of a form that ensures conjugate action when forged gear 205 is meshingly engaged with a not shown pinion gear.

Tooth tip 234 has a smooth non-planar form that intersects with tooth flank 209 in line 233. Tooth tip 234 also intersects with tooth outer end 213 in line 235. Tooth tip 234 also intersects with not a shown tooth inner end 212 (corresponding to inner tooth end 12 in FIG. 1) in a not shown line 236 which is similar in form to line 235 in view B of FIG. 5 a.

Forged gear tooth 205 is formed by urging a not shown forging blank (preform) into tooth die cavity 237 such that at the end of the forging process there is no metal-to-metal contact between gear tooth tip 234 and the forging die cavity 237. This is achieved by designing the not shown forged gear blank preform shape, the forging process lubrication conditions and the forging die apparatus to ensure that tooth die cavity 237 fills evenly such that at the end of the forging stroke curved surface 238 of tooth tip 234 is substantially parallel with root cavity surface 242 for the entire length of the die tooth cavity, but does not contact it.

The normal pressure acting on the surface of gear tooth tip 234 throughout the forging stroke is equal to the air pressure in the die cavity, which for all practical purposes may be considered to be zero in comparison with the normal pressures acting on the die surfaces which are in contact with the material being forged. By ensuring that there is no metal-to-metal contact between the gear tooth tip 234 and the bottom of the die cavity 242 during forging, the forging pressure is thus minimized and hence the corresponding die pressures and stresses are minimised for a given die geometry. To ensure the pressure in the bottom of the die cavity is negligible compared to the material internal pressure, it is necessary to ensure there is no fluid die lubricant trapped in the die cavity during forging.

The provision of a low pressure zone along the full extent of the gear tooth tip 234 in the die cavity acts in a way similar to that of a flash gutter in open die forging. The low pressure zone limits the forging pressures in the die cavity to significantly lower values than would be achieved if the die cavity were to be fully filled with metal-to-metal contact established everywhere in the die cavity. In this way the least possible die forging stress outcomes may be realised for a given die design.

Such a low pressure zone may be achieved by limiting the fill of the tooth die cavity, in which case said cavity will be the obverse of the required gear tooth form except for the root region which needs to be machined to a depth in excess of the height of the finish forged gear tooth in order to create the necessary clearance space above the gear tooth tip when the die is fully closed. Alternatively, one could omit the root portions of the tooth die cavities altogether, thus creating an open die cavity above the tip of each gear tooth.

FIGS. 5 b and 5 c depict clearance 239 that exists between tooth tip 234 and die cavity 237 at the end of the forging process.

FIG. 5 d shows curved surface 238 of tooth tip 234 that is formed when gear tooth 205 is forged in such a way that tooth tip 234 does not come into contact with the tooth die root cavity surface 243 of die cavity 237. Curved surface 238 extends over the entire length of tooth tip 234, along curvilinear direction t.

The cross-sectional shape of tooth die root cavity surface 243 is defined by tooth die root cavity radii 240 and 241 and tooth die bottom surface 243, seen in end view in FIG. 5 d. In order to minimise die stress concentration, die root cavity radii 240 and 241 should be made as large as possible in which case the area of tooth die bottom surface 243 may be reduced to zero and tooth die root cavity radii 240 and 241 then become one continuous, though not necessarily constant, radius. The maximum allowable size of root radii 240 and 241 will depend on the vertical extent of the not shown contact zones on the flanks 209 of tooth 205.

FIG. 5 e shows tooth die cavity outer end root radius 244 and tooth die outer end root cavity 247. Again, in order to minimise die stress concentration, tooth die cavity outer end root radius 244 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 240 and 241.

FIG. 5 f shows tooth die cavity inner end root radii 245 and 246 and tooth die outer end root cavity 248. Again, in order to minimise die stress concentration, tooth die cavity inner end root radii 245 and 246 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 240 and 241.

As shown in FIG. 5 c the profile of tooth tip 234 is flat except at the ends of the tooth where it blends with the tooth ends. However, this will generally not be the case for a forged gear tooth. Depending on the design of the not shown preform and the lubrication conditions, the profile of tooth tip 234 will normally be curved.

FIG. 5 g depicts a second embodiment where tooth tip 234 has a convex crowned portion 260 extending the full length of tip 234 along curvilinear direction t.

In the context of this specification, crowning is defined as the convex upward profile of tooth tip 234 when viewed in direction C of FIG. 5 a. To facilitate comparison FIG. 5 g and not yet discussed FIGS. 6 k, 7 d and 7 e are shown together on the same page.

FIG. 6 a shows a third embodiment of a forged tooth 305 which is a variation on forged tooth 205 of the first embodiment. In this third embodiment tooth tip 334 has been allowed to come into contact with the bottom of tooth die cavity 337 over tip contact area 370. In this case, the pressure acting on tip contact area 370 at the end of the forging process will be non-zero, and the corresponding die pressures and stresses experienced in forming forged tooth 305 will be higher than those experienced in forming forged tooth 205. Whether the die cavity fills uniformly without contacting the root of the tooth die cavity depends on the design of the not shown forged gear preform, the die lubrication conditions and the extent to which the die cavity is filled during forging. To keep mechanical stresses in the tooth die to acceptable levels during forging, the length 350 of tip contact area 370 preferably should not exceed 50% of the length of tooth tip 334.

The location of tip contact area 370 is preferably in the centre of tooth tip 334 but it may be closer to one end than the other provided area 370 does not extend into a corner of the tooth die cavity. If tip contact area 370 extends into a corner, the tooth material may fill this corner of the die cavity first, and this may promote an unacceptably high hydrostatic pressure in the die cavity with consequent high die stresses. The location of tip contact area 370 is influenced by the design of the tooth die cavity, the forged gear preform and the die lubrication conditions.

FIGS. 6 b and 6 c show the partial clearance 339 that exists between the tooth tip 334 and the root of die cavity 337 at the end of the forging process.

FIG. 6 d shows curved surface 338 of tooth tip 334 that is formed when gear tooth 305 is forged in such a way that tooth tip 334 partially contacts the tooth die root cavity surface 342 of die cavity 337. Curved surface 338 extend substantially over the length of tooth tip 334, in the curvilinear direction t shown in FIG. 1.

The cross-sectional shape of tooth die root cavity surface 342 is defined by tooth die root cavity radii 340 and 341 and tooth die bottom surface 343, seen in end view in FIG. 6 d. In order to minimise die stress concentrations, die root cavity radii 340 and 341 should be made as large as possible in which case the area of tooth die bottom surface 343 may be reduced zero and tooth die root cavity radii 340 and 341 then become one continuous, though not necessarily constant, radius. The maximum allowable size of root radii 340 and 341 will depend on the vertical extent of the not shown contact zones on the flanks 309 of tooth 305.

FIG. 6 e shows tooth die cavity outer end root radius 344 and tooth die outer end root cavity 347. Again, in order to minimise die stress concentration, tooth die cavity outer end root radius 344 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 340 and 341.

FIG. 6 f shows tooth die cavity inner end root radii 345 and 346 and tooth die outer end root cavity 348. Again, in order to minimise die stress concentration, tooth die cavity inner end root radii 345 and 346 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 340 and 341.

FIG. 6 k shows tooth tip 334 with convex crowned portions 360 and 361 juxtaposed with a substantially centrally located portion 362 that may be formed either by contact with a not shown tooth die during forging or by a subsequent machining operation. Portion 362 may be either substantially flat, slightly rounded or rounded depending on the design of the die root cavity or the tooth tips have been machined after forging.

FIG. 6 g shows a fourth embodiment of a forged tooth 705 in which tooth tip 734 is allowed to extend outside the envelope of the parent machine cut gear tooth tip 11, here indicated by tooth tip 711. This is because the conical (approximately flat) surface of tooth tip 11 of parent machine cut tooth 5 may be replaced, when designing the corresponding tooth die cavity, by curved surface 738 that extends outside the envelope of parent machine cut tooth 5 provided curved surface 738 does not interfere with its not shown conjugate meshing gear. If curved surface 738 causes interference, then a portion 749 of tooth tip 734 may be removed by machining after forging. Although this introduces an extra operation and wastes material, this may be preferable if one is unable to achieve acceptable die life using small corner radii 740 and 741 in an endeavour to keep the forged tooth form within the envelope of the parent machine cut tooth 5.

The shape of curved surface 738 is similar to that of curved surface 238 in that it is formed without the tip 734 coming into contact with the surface of die cavity 737, thus leaving clearance 739 at the end of the forging stroke. This is achieved by cutting die cavity 737 deeper than die cavity 237 and designing the not shown forged gear preform, the forging process lubrication conditions and the forging apparatus to ensure that tooth die cavity 737 fills evenly such that at the end of the forging stroke curved surface 738 of tooth tip 734 is substantially parallel with and does not contact root cavity surface 742 for the length of the die tooth cavity, the length being measured in curvilinear direction t.

The cross-sectional shape of tooth die root cavity surface 742 is defined by tooth die root cavity radii 740 and 741 and tooth die bottom surface 743, seen in end view in FIG. 6 g. In order to minimise die stress concentration, die root cavity radii 740 and 741 should be as large as possible in which case the area of tooth die bottom surface 743 may be reduced zero and tooth die root cavity radii 740 and 741 then become one continuous, though not necessarily constant, radius. The maximum allowable size of root radii 740 and 741 will depend on the vertical extent of the not shown contact zones on the flanks 709 of tooth 705.

FIG. 6 h shows tooth die cavity outer end root radius 744 and tooth die outer end root cavity 747. Again, in order to minimise die stress concentration, tooth die cavity outer end root radius 744 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 740 and 741.

FIG. 6 j shows tooth die cavity inner end root radii 745 and 746 and tooth die outer end root cavity 748. Again, in order to minimise die stress concentration, tooth die cavity inner end root radii 745 and 746 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 740 and 741.

FIG. 7 a depicts a fifth embodiment of a forged tooth 405 which is a further variation of forged tooth 205 of the first embodiment. In this embodiment the tooth tip 434 has been allowed to contact the root of tooth die cavity 437 over areas 442 and 443 having lengths 444 and 445 respectively. In this case, the pressures acting on areas 442 and 443 at the end of the forging process will be non-zero, and the corresponding die pressures and stresses experienced in forming forged tooth 405 will be higher than those experienced in forming forged tooth 205. To keep mechanical stresses in the tooth die to acceptable levels during forging, the lengths 444 and 445 of areas 442 and 443 should be similar and their combined length should not exceed 50% of the length of tooth tip 434.

Note that the locations of areas 442 and 443 are preferably symmetrically disposed about the centre of tooth tip 434 but they may both be closer to one end than the other provided they do not extend into a corner of the tooth die cavity. If area either of 442 or 443 extends into a corner radius, the tooth will fill the corner of the die cavity first, and this may promote an unacceptably high hydrostatic pressure in the die cavity with consequent high die stresses. The location of areas 442 and 443 is influenced by the design of the tooth die cavity, the forged gear preform and the die lubrication conditions.

FIGS. 7 b and 7 c depict views B and C of FIG. 7 a respectively, and show the partial clearances 439 that exist between the tooth tip 434 and the root of die cavity 437 at the end of the forging process. Note that it is permissible for the tip 434 of forged tooth 405 to protrude slightly above the surface of tooth tip 411, for the same reason as earlier given for forged tooth form 305.

Note that the detail in view D in FIG. 7 b is the same as that of view D in FIG. 6 b.

Note that the details in views E and F of FIG. 7 c are same as those of views E and F in FIG. 6 c.

FIG. 7 e shows tooth tip 434 with crowned portions 460 and 461 juxtaposed with flat tooth tip portions 442 and 443 that are in turn juxtaposed with a substantially centrally located concave tip portion 462. FIG. 7 e illustrates the case where tooth tip 434 has either contacted the not shown tooth die in two places during forging or where tooth tip 434 has been machined after forging to form the two substantially flat tooth tip portions 442 and 443.

FIG. 7 d shows a sixth embodiment of a tooth tip 634 with convex crowned tooth tip portions 660 and 661 juxtaposed with a substantially centrally located concave tooth tip portion 662. FIG. 7 d illustrates the case where tooth tip 634 has not come into contact with a not shown tooth die during forging, in which case there will be no tip contact areas formed on the tips of crowned portions 660 and 661.

Provided forged teeth 205, 305, 405 and 705 substantially resemble the form of the parent machine cut gear tooth 5, all other things being equal, the lowest die stress outcome will be achieved with forged teeth of the form of teeth 205 and 705. Whether the die stress outcome for forged tooth 305 is greater or less than that of forged tooth 405 will depend on subtle variations in the design of the forged gear preform.

However, a further and most significant reduction in forge die stress over that achievable with forged tooth form 205 is possible if the form of the forged tooth is no longer constrained to be substantially that of the parent machine cut gear tooth. Such a tooth form is depicted in FIG. 8 a as a seventh embodiment of the present invention.

In FIG. 8 a, the outline of the parent machine cut gear tooth 5 is indicated by the chain dotted lines 515, 516, 517, 518, 519, 520, 521, 522, 523, 524, 525, 528, 529, 530, 532 and the solid lines 526, which represent the locus of the lowest points in root radii 514.

Let the boundary of contact zone 509 be delineated by curved boundary 548. In FIG. 8 a the tooth flank shown is convex and is known in the art as the “drive flank” because it is the flank that sustains the major axle driving loads. The not shown concave opposite tooth flank is known in the art as the “coast flank” and is subjected to lower loads than the drive flank.

When gear tooth 505 is lightly loaded, the extent of the contact zone is delineated by curved boundary 576. When the gear is heavily loaded, the contact zone moves towards outer end 513 of gear tooth 505 and is delineated by boundary 577. Curved boundary 548 encloses the contact zones bounded by curved boundaries 576 and 577 but does not intersect them. To provide for good meshing contact between the shown gear and the not shown meshing pinion, there will necessarily be a small clearance between boundary 548 and boundaries 576 and 577. Further, to ensure conjugate action between forged gear 505 and its not shown meshing pinion, the surface delineated by curved boundary 548 must be conjugate to the mating pinion. However, the form of the surface of gear tooth 505 outside the boundaries of the conjugate contact zones on the two tooth flanks may be any shape acceptable to both the gear designer and the tooth die designer.

FIG. 8 a shows a smooth curved forged tooth form 505 that would be preferred by a tooth die designer over those of forged teeth 205, 305, 405 and 705. This is because the not shown tooth die cavity corresponding to the smooth curved gear tooth 505 shown in FIG. 8 a permits more generous radii in the corners and the root region of the tooth die cavities as well as around the top edges of the tooth die cavities. The not shown tooth die cavity end corner and root radii correspond to radii 540, 541, 544, 545 and 546 of FIGS. 8 d, 8 e and 8 f. The not shown tooth die cavity top edge radii are the obverse of radii 514, 550 and 551 in FIGS. 8 b and 8 c. Provision of these generous radii in the design of the tooth die cavity results in reduced stress concentrations in the forging die tooth cavities, which contribute directly to reduced die stresses and substantially increased die life.

The larger radii at the top edges of tooth die cavities based on the form of gear tooth 505 tooth also facilitates the flow of metal into the tooth die cavity, reducing the material hydrostatic pressure which results in a further reduction in forging die stresses and further enhances die life.

FIG. 8 d shows curved surface 538 of tooth tip 534 that is formed when gear tooth 505 is forged in such a way that tooth tip 534 does not come into contact with the tooth die root cavity surface 542 of die cavity 537. Curved surface 538 extends substantially over the length of tooth tip 534, in the curvilinear direction t shown in FIG. 1.

The cross-sectional shape of tooth die root cavity surface 542 is defined by tooth die root cavity radii 540 and 541 and tooth die bottom surface 543, seen in end view in FIG. 8 d. In order to minimise die stress concentration, die root cavity radii 540 and 541 should be as large as possible in which case the area of tooth die bottom surface 543 may be (desirably) reduced zero and tooth die root cavity radii 540 and 541 then become one continuous, though not necessarily constant, radius. The maximum allowable size of root radii 540 and 541 will depend on the vertical extent of the not shown contact zones on the flanks of tooth 505.

FIG. 8 e shows tooth die cavity outer end root radius 544 and tooth die outer end root cavity 547. Again, in order to minimise die stress concentration, tooth die cavity outer end root radius 544 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 540 and 541.

FIG. 8 f shows tooth die cavity inner end root radii 545 and 546 and tooth die outer end root cavity 548. Again, in order to minimise die stress concentration, tooth die cavity inner end root radii 545 and 546 should be made as large as possible within the constraints of the gear design and should blend smoothly with tooth die root cavity radii 540 and 541.

FIG. 8 g depicts sections (i), (ii), (iii), (iv) and (v) of FIG. 8 a, and shows the cross sections of forged tooth 505 compared to the cross sections of the parent machine cut gear tooth 5 at the same locations. A forged gear tooth of the form of gear tooth 505 results in mass saving and die stress reductions when compared to gear tooth form 5.

FIG. 8 h shows views G,H,J,K and L of FIG. 8 g. Views G, H and J show clearance 539 that exists between tooth tip 534 and die root cavity surface 542. Views K and L show that clearance 539 has reduced to zero towards the ends of the tooth.

Clearance 539 shown in FIGS. 8 d, 8 e and 8 f reflects the fact that the die cavity 537, the not shown forged gear perform and the die lubrication conditions are designed to ensure that, as for tooth forms 205 and 705, during forging there is no contact between the tooth tip 534 and the die root cavity surface 543 over a substantial portion of the axial length of the gear tooth.

In addition to substantially reducing forging die stresses, crowning the tooth tips in the longitudinal direction in the manner earlier described permits the use of a preform of lesser mass, resulting in a material saving. The amount of material saved is maximum when teeth of the form of tooth 505 are forged. A smaller material saving is also possible if teeth of the form of 205, 305 and 405 are forged with crowned tips instead of flat tips. Although the amount of material saved is less in these latter cases, it is not negligible given the large number of gears manufactured each year by the automotive industry globally.

A further benefit of gear teeth forged with longitudinally crowned teeth and with smooth corner radii is a reduction in the tendency to aerate the lubricating oil over that achieved by sharp edged gear teeth. The generous tooth tip edge radii promote a smooth flow of oil over the gear teeth surfaces as they enter the lubricant pool, in contrast with that of sharp edged gear teeth that can cause cavitation and air entrainment in high velocity applications.

A further benefit of gear teeth forged with longitudinally crowned teeth having smooth corner, edge and root radii is the improvement in workplace safety that results from the elimination of the sharp corners that are inevitably associated with machine cut gear teeth.

A yet further benefit of a gear having longitudinally crowned teeth together with smoothly blended corner, end and root radii is the flexibility afforded both the gear designer and the tooth die designer. Such tooth forms are easy to produce by forging whereas they are costly to produce by machining. No longer is one constrained to work with the tooth forms dictated by gear cutting, and it becomes possible to evolve new tooth forms having desirable strength and performance characteristics that can be formed in a cost effective forging process with unprecedented die life.

FIG. 9 a depicts a perspective view of a hypoid ring gear 1 with machine cut teeth of the form of gear tooth 5 in FIG. 4 a. FIG. 9 b depicts a perspective view of a hypoid ring gear 701 with forged teeth of the smooth form of gear tooth 505 in FIG. 8 a. The parent gear tooth form for ring gear 701 is gear tooth form 5 in FIG. 4 a, that is the size and location of the contact zones in gears 1 and 701 are substantially the same in extent and location, and gear teeth are bounded by the form of gear tooth 5, except that forged tooth tips 234, 334 and 434 may extent slightly beyond the boundary of tip surface 11 in FIG. 4 a or the ends 212 and 213 in FIG. 4 a provided this does not cause interference with the mating not shown gear and other components in the axle assembly. In this way we may say that the tooth form is substantially bounded by the machine cut tooth form, it being understood that it is only at the tips and the ends of the teeth where we may go slightly outside the boundary of the parent machine cut tooth form. The flanks and particularly the contact zones must always remain bounded by the machine cut gear tooth form to ensure conjugate action with the mating not shown gear.

Ring gears with forged gear teeth based on the smooth curved tooth form 505 shown in FIG. 8 a will, all other things being equal, yield the lowest possible die mechanical stresses with attendant increase in die fatigue life.

In highly loaded drive applications, the extra demands for tooth bending strength and rigidity may necessitate a tooth form closer to that of the parent machine cut gear. In such cases, the limits size of the tooth form will lie somewhere between that of tooth 205 and tooth 505.

In general, the lowest die stress outcome will be obtained if the die and the gear preform is designed to produce the tooth form where the tooth tip does not contact the bottom of the tooth die cavity at any point along the length of the tooth. The lowest die stress outcome will be achieved with tooth form 505.

The next lowest die stress outcome will be achieved with tooth 205, or a tooth having a form lying somewhere between the limits of form of tooth 205 and 505, provided there is no contact between the tip of the tooth and the bottom of the tooth die cavity during forging.

Forged gear teeth based on the tooth forms 305 and 405 shown in FIGS. 6 a and 6 a may be required if at least one of the contact zones of necessity extends close to tip of the gear tooth. In such cases, satisfactory stress outcomes can be still be achieved provided the contact between the tip of the die tooth and the corresponding root in the tooth die cavity is limited to no more than 50% of the tooth tip length.

Although the present invention has been described with reference to the design and manufacture by forging of a bevel gear and in particular a hypoid bevel gear, those knowledgeable in the art will appreciate that the principles disclosed herein may be applied to the design and manufacture by forging of geared components having other than hypoid gear teeth without departing from the spirit of the invention. For example, the longitudinal extent of the bevel gear tooth is described in this specification as the “radial direction”, as shown in FIGS. 1 and 3. For spur and helical gears the longitudinal extent of the gear tooth is the axial direction ie a direction parallel to the rotational axis. For rack gears the longitudinal extent of the gear tooth is the transverse direction ie a direction at substantially right angles to the translational axis of the rack. 

1. A forged bevel gear having a plurality of teeth, each tooth having a tip disposed between two faces and two ends, said two faces extending in a substantially longitudinal direction of said tooth, characterised in that said tip has at least one substantially crowned portion between said two ends along said longitudinal direction.
 2. A forged gear as claimed in claim 1, wherein said crowned portion is forged as a smooth continuous surface.
 3. A forged gear as claimed in claim 1, wherein said substantially longitudinal direction of said tooth lies along a substantially radial direction.
 4. A forged gear as claimed in claim 3, wherein said radial direction is curvilinear.
 5. A forged gear as claimed in claim 3, wherein said radial direction is straight.
 6. A forged gear as claimed in claim 2, wherein said at least one crowned portion is two crowned portions.
 7. A forged gear as claimed in claim 6, wherein a central portion is located between said two crowned portions, and said central portion is any one of a substantially flat portion, a slightly rounded portion or a rounded portion.
 8. A forged gear as claimed in claim 6, wherein a substantially concave portion is centrally located between said two crowned portions.
 9. A forged gear as claimed in claim 1, wherein at least one flat is machined onto said gear between said at least one substantially crowned portion and one of said ends.
 10. A forged gear as claimed in claim 1, wherein at least 25% of the length of said tip is crowned.
 11. A forged gear as claimed in claim 1, wherein at least 50% of the length of said tip is crowned.
 12. A method of forging a gear from a blank, said gear having a plurality of teeth, each said tooth having a tip disposed between two faces and two ends, said two faces extending in a substantially longitudinal direction of said tooth, said method comprising the steps of: i) placing said blank within a die means comprising a cavity having a shape that is the near obverse of said teeth; and ii) forging said blank within said cavity to form said teeth thereon; characterised in that during said forging, a substantial portion of the length of each said tip does not contact said cavity.
 13. A method of forging a gear as claimed in claim 12, wherein said substantial portion is at least fifty percent of the length of said tip.
 14. A method of forging a gear as claimed in claim 12, wherein said substantial portion is at least sixty percent of the length of said tip.
 15. A method of forging a gear as claimed in claim 12, wherein said substantial portion is at least seventy-five percent of the length of said tip.
 16. A method of forging a gear as claimed in claim 12, wherein said tip has at least one substantially crowned portion between said two ends along said longitudinal direction.
 17. A method of forging a gear as claimed in claim 12 wherein said gear is a bevel gear.
 18. A forged gear manufactured by the method of claim
 12. 19. A forged bevel gear having a hypoid configuration for use in a vehicle power transmission, said gear comprising a plurality of teeth, each tooth having a tip disposed between two faces and two ends, said two faces extending in a substantially radial direction, characterised in that said tip has at least one substantially crowned portion between said two ends along said radial direction.
 20. A forged gear manufactured by the method of claim
 13. 